(1) Field of the Invention
The present invention relates to an optical communication system and an optical receiver.
(2) Description of Related Art
In the recent years, the employment of a large-scale integrated circuit (LSI) has permitted an error correction code to be used as a transmission code. At present, an LSI capable of correcting and reducing an error rate of 2×10−4 up to an error rate of approximately 10−15 has been put to practical use. And not only that, an LSI provided with an error correction code for making correction from 1×10−2 to 10−15 has been in development.
Naturally, in the case of the employment of such a high correction capability code, an optical receiver would be required to operate normally in poor S/N conditions causing an error rate of 1×10−2.
So far, the optical receiver has conducted an operation to make up an input to an identification unit to a constant amplitude for normal identification. As this approach, there has been employed a structural example in which a peak value of an equalized waveform signal after amplification is detected to give feedback (see FIG. 6) or a structural example in which an optical output power is made constant by an optical amplifier (see FIG. 7).
In FIG. 6, reference numeral 1 represents a light-receiving device (PD: PhotoDiode), numeral 2 represents a gain variable type preamplifier [AGC (Automatic Gain Controlled) amplifier], numeral 3 represents a low-pass filter (LPF), numeral 4 designates a band-pass filter (BPF), numeral 5 designates a differential amplifier, numeral 6 designates a flip-flop circuit (FF), numeral 7 depicts a peak value detecting circuit, numeral 8 depicts a limiter amplifier, numeral 9 depicts a signal identifying section, numeral 10 denotes a buffer amplifier, and numeral 18 denotes a clock extracting circuit.
In the optical receiver shown in FIG. 6, a PD 1 comprising a PIN photodiode or an avalanche photodiode receives light from an optical transmission line, and the preamplifier 2 amplifies the output of the PD 1. The output of the preamplifier 2 is inputted to the low-pass filter 2 and further to band-pass filter 4. The low-pass filter 3 is a filter having 3-dB down band corresponding to 0.7 to 0.8 of the bit rate of a received signal. The output of the low-pass filter 3 is inputted to the differential amplifier 5 and further to the peak value detecting circuit 7.
The peak value detecting circuit 7 is for detecting a peak value of a signal (received data signal) inputted to the differential amplifier 5 of the signal identifying section 9 and adjusting the gain of the preamplifier 2 to make constant the signal level inputted to the signal identifying section 9. The differential amplifier 5 of the signal identifying section 9 is for shaping a signal waveform in its amplitude direction by applying a value minimizing the error rate as a reference voltage Vref.
The band-pass filter 4 of the clock extracting circuit 18 is a filter for clock extraction, which detects a frequency (clock component) corresponding to the bit rate of a data signal. In this connection, the clock extracting circuit 18 shown in FIG. 6 has an arrangement for when the signal to be sent from the optical transmission line is an RZ (Return to Zero) signal. In the case of an NRZ (Non-Return to Zero), since a clock component does not exist in the signal spectrum unlike the RZ signal, there is a need to place a differentiating circuit for detecting an edge of a signal and a rectifying circuit for rectifying the output of the differentiating circuit in the former stage of the band-pass filter 4 to generate a clock for signal identification.
The output of the band-pass filter 4 is inputted to the limiter amplifier 8, and the limiter amplifier 8 cuts off both a peak level and bottom level of the output of the band-pass filter 4 and amplifies the received signal so that its upper limit and its lower limit assume predetermined values, respectively, for clock production. The clock thus shaped in the limiter amplifier 8 is inputted to the flip-flop circuit 6 of the signal identifying section 9. The flip-flop circuit 6 punches the output of the differential amplifier 5 with the output of the limiter amplifier 8 for outputting data subjected to signal identification. The buffer amplifier 10 is for setting up the circuit isolation.
On the other hand, in a configuration shown in FIG. 7, an optical amplifier 11, such as an EDF (Erbium Doped Fiber) amplifier, is provided at the final stage of an optical transmission line. In the FIG. 7 configuration, the same parts as those in FIG. 6 are marked with the same reference numerals. This optical amplifier 11 is designed to implement constant output control so that the signal power to be inputted to the PD 1 is made constant at all times.
Accordingly, a preamplifier 2′ at the latter stage is required to perform simple amplification only, and this eliminates the need for the feedback control based on the received signal peak value detection in the FIG. 6 configuration.
Meanwhile, the received waveform in the above-mentioned optical receiver varies, for example, as shown in FIGS. 8A to 8C according to optical S/N. FIG. 8A shows a received waveform (eye pattern) in a case in which the SIN is at 8 dB (error rate=1×10−1), FIG. 8B shows a received waveform in a case in which the SIN is at 17.5 dB (error rate=1×10−4), and FIG. 8C shows a received waveform in a case in which the S/N is at 21.5 dB (error rate=1×10−9).
For example, in the poor optical S/N condition shown in FIG. 8A, in an optical receiver with the configuration shown in FIG. 6, the peak value detecting circuit 7 detects a peak value of a noise instead of a peak value of the intended received signal (data signal), and the amplitude of the received signal (data signal) inputted to the signal identifying section 9 becomes lower than the proper amplitude. For this reason, the signal identification indeterminate factors increase so that the actual error rate is impaired as compared with the error rate based on the optical S/N. Moreover, since the amplitude inputted to the clock extracting circuit 18 decreases, it tends to be out of the dynamic range of the clock extracting circuit 18.
In the poorer optical S/N condition, the ASE (Amplified Spontaneous Emission) light increases, which decreases the optical signal component with respect to the entire light. With this fact, in the case of the FIG. 7 configuration of the optical receiver which implements the constant output control on the entire light in the optical amplifier 11, similarly, the amplitude of the received signal to be inputted to the signal identifying section 9 becomes lower and the amplitude to be inputted to the clock extracting circuit 18 also becomes lower, which creates the same problem as that of the optical receiver with the configuration shown in FIG. 6.
In addition, although the error correction code is capable of correcting an error of a data signal, it is helpless against a case in which a clock signal falls into a malfunctioning condition. Although simple clock extracting circuits using a PLL circuit have frequently been put to use, if the optical S/N is in a poor condition, since a large noise is introduced into a phase comparator constituting the PLL circuit, the phase jump of the clock occurs to cause the out-of-synchronization, thereby producing huge burst errors.
For these reasons, the conventional optical receivers shown in FIGS. 6 and 7 do not function normally when a received signal is in a low S/N condition.